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Clay shows promise in developing high-temp batteries

Posted: 19 Nov 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Rice University? Li-ion batteries? electrolyte? clay? defence?

A team of researchers at Rice University has come up with a unique combination of materials, which include a clay-based electrolyte, to solve a problem for rechargeable Li-ion batteries intended for harsh conditions. According to the scientists, the Li-ion chemistry-based battery is robust enough to supply stable electrochemical power in temperatures up to 120C, where they can be used in space, defence and oil and gas applications, among others.

Chemist Pulickel Ajayan and his colleagues at Rice and at Wayne State University in Detroit described the material this month in the American Chemical Society journal ACS Applied Materials and Interfaces.

This discovery, like earlier work on supercapacitors by the lab, depends on the malleable qualities of bentonite clay and room-temperature ionic liquids that serve as both a separator and an electrolyte system and provide a conductive path between a battery's anode and cathode.

Kaushik Kalaga

Kaushik Kalaga spreads a clay-based electrolyte/separator on one half of a button battery for testing. The batteries are meant for high-temperature environments where present Li-ion batteries cannot be used. (Photo by Jeff Fitlow)

"Clay naturally has a lot of moisture in it, and that's not a problem when you're doing supercapacitors," said Kaushik Kalaga, a graduate student in Ajayan's lab and lead author of the study. "But a battery has to have a lithium-ion conductive species in the electrolyte to conduct lithium ions from the cathode or anode, or vice versa, when you charge and discharge.

"Lithium is very reactive with water, so our first challenge was to eliminate water from the clay while keeping its structure intact," he said.

Kalaga and his team started by baking commercial clay particles at 650C for an hour to dry them out. They then combined a room-temperature ionic liquid and lithium salt and mixed them into the clay in an oxygen-free glove box. The liquefied salt acts as a source of lithium ions that conduct through the electrolyte to the electrodes.

Clay-based compound

A clay-based compound invented at Rice University is an electrolyte and a separator for Li-ion batteries for use in high-temperature environments. (Photo by Jeff Fitlow)

The researchers spread the resulting peanut butter-like slurry between lithium metal electrodes and encapsulated them in coin-shaped batteries for testing at various temperatures.

Conventional organic electrolytes cannot be used in batteries over 60C, due to their low boiling temperature; the vapours that form beyond 80C can lead to an explosion, Kalaga said. Batteries that have solid-state electrolytes work in high temperatures, but the electrolytes don't connect as well with electrodes, which hurts performance.

The researchers built their composite electrolyte to be tough and conductive and still present the maximum surface area to electrodes to provide a solid path for current.

The units proved able to deliver current at high temperatures with a stable voltage window of 3V over 120 charge-discharge cycles and featured both the thermal stability of solid-state electrolytes and the wetting properties of liquid electrolytes, assuring good contact with the electrodes. The voltage window is the range between which the electrolyte is stable and is not chemically degraded.

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